In modern vehicles, and notably in aircraft, the piloting/steering of the vehicle calls upon numerous operating levers, control handles and other control elements which may be subjected sometimes to a manual control implemented by a human pilot, sometimes to an automatic control worked out by an autopilot system, and at times to both simultaneously.
For example, a throttle lever of an aircraft is constituted in general by a lever pivoting about an axis in a sagittal plane of the aircraft between an extreme angular position in which the lever is oriented towards the rear of the plane, this position corresponding to a minimum of thrust of the engines, and an opposite extreme angular position corresponding to the maximal thrust. Of course, other movements—for example, translation, translation along a curve, etc.—are possible, according to the kinematic linkage installed. In an aircraft equipped with an autopilot system the thrust of the engines can be controlled by this system in accordance with previously established programming. However, in order to give information to the pilot about the thrust demanded by the autopilot system it is common practice to equip the lever with an electric motor driving an appropriate kinematic linkage aiming to displace the lever between its extreme positions, in order to reflect the position corresponding to the thrust demanded. Such a device is called an autothrottle.
Furthermore, when the autopilot system is deactivated the pilot can manipulate the operating lever so as to define the appropriate thrust in the current flight phase. In the case of aircraft equipped with electric controls (‘fly by wire’), it is necessary to add a friction brake to the operating lever, in order to provide the pilot with a sensation of resisting force analogous to that caused by the friction of the linkage of the mechanical throttle levers and/or of the cables used previously.
However, in certain emergency cases the pilot is induced to manipulate the throttle lever without having deactivated the autopilot system, and may find himself/herself confronted with a situation in which he/she has to exert an inordinate force on the lever in order to overcome the control exerted by the electric motor acting on the throttle lever. It is therefore necessary to provide a limitation of the force necessary for the pilot to take over control from the autopilot system.
From patent U.S. Pat. No. 5,613,652, for example, a throttle lever of an aircraft is known which is driven in rotation by a servomotor controlled by a closed-loop control of the position of the lever. The servomotor is linked to the lever of the throttle by means of a friction clutch so as to enable the regaining of control of the throttle lever by the pilot without said pilot having to exert an excessive force in order to counter the control of the servomotor. The force necessary to overcome the control of the servomotor is thus limited to the resistance opposed by the friction of the clutch.
However, there are certain cases in which it is preferable that the electric motor is directly connected to the lever, for example in order to avoid a situation where the measurement of the position of the motor and of the lever are conflicting on account of the sliding of the clutch, or in order to enable a programming of notches or of active hard points that are suitable to enable a return of tactile position to the pilot.
In particular, in the case of a throttle lever having a displacement that is relatively slow, use is currently made of an electric motor of synchronous type, preferably with permanent magnets, which enables an excellent power-to-weight ratio to be obtained, produces a better output than the direct-current motor, and does not use brushes, thus avoiding wearing parts.
A synchronous motor comprises, from the mechanical point of view, a fixed part, the stator, and a part that is mobile in rotation about an axis, the rotor. The synchronous motor also comprises, from the electrical point of view, an inductor which generates the magnetic field enabling the operation of the motor, and an armature which, flowed through by a current, generates the torque and therefore the rotation of the motor. The inductor may be realised with permanent magnets (the case of the synchronous motor with permanent magnets) or by one or more coils flowed through by a direct current. Moreover, the inductor or the armature may be alternatively fixed or mobile—that is to say, stator or rotor. In the present text, by way of example and for reasons of simplicity, use is made of a synchronous motor with permanent magnets, the inductor of which, constituted by magnets, forms the rotor, and the armature of which, comprising windings, forms the stator. Consequently, since the inductor does not comprise coils, it is advisable to understand “winding of the armature of the motor” when reference is made, by simplification, to “winding of the motor”. It is, however, possible to use any other variant, such as a wound inductor rotor and an armature on the stator, etc.
These synchronous motors, however, necessitate a more complex control than that of the direct-current motor, obtained by the implementation of electronic means. More generally, a motor assembly comprises the actual motor, with a stator forming the armature, preferably wound in three-phase manner (one winding per phase), and an inductor rotor, preferably with permanent magnets, a converter or inverter feeding each of the windings of the stator from the direct current, and a position sensor informing a control computer about the position of the motor.
Several control modes of the motor may be employed, such as, for example, the so-called six-state simple control, in which each of the windings of the motor is fed as a function of the angular position of the motor by gating pulses of voltage: positive constant voltage over 120°, zero voltage over 60°, negative constant voltage over the following 120°, then zero voltage over the last 60° for one of the windings, the feed of the two other windings being deduced therefrom by a phase shift of 120°.
Another control mode, so-called scalar control, consists in feeding each winding with a sinusoidal alternating voltage, generated for example by an inverter as a function of the position of the motor.
However, the preferred control mode of this type of motor remains the so-called ‘vectorial’ control, in which a control computer includes a part for vectorial control of the inverter, defining the frequency and the amplitude of the feed of the windings of the stator on the basis of the position of the motor and on the basis of control values of a first voltage, the so-called direct voltage Vd, and of a second voltage, the so-called quadratic voltage Vq, by employing mathematical transformations known under the name of PARK and CONCORDIA transforms. These transformations are implemented in a first vectorial-transformer block which transforms the two values of direct voltage and quadratic voltage into controls of the inverter, and in a second, inverse-vectorial-transformer block which, on the basis of controls applied by the inverter, provides a value of a first current, the so-called direct current Id, and of a second current, the so-called quadratic current Iq.
Whatever the control mode of the motor, the control computer also includes a part for determination of the control parameters, for example the voltage of the gating pulses for the six-state control, or the direct and quadratic voltages for vectorial control on the basis of set points for operation of the motor, such as, for example, its position or its speed of rotation or its torque.
Conventionally, a process for speed regulation of a synchronous motor with permanent magnets includes a first closed loop for regulation, receiving a set point for speed of rotation of the motor, a comparator for comparing this set point with a measurement of the speed of rotation of the motor, for example by differentiating the information about position of the motor with respect to time, and a regulator that is suitable to deduce a set-point torque of the motor from the speed error. The regulation includes a second closed loop for interleaved regulation, in which the set-point torque is compared to a measurement or an estimation of the instantaneous torque provided by the motor, and the torque error is transformed by a second regulator into a control value of at least one of the control parameters of the motor.
Usually the estimation of the instantaneous torque provided by the motor is realised on the basis of the measurement of the current consumed by said motor, for example by measuring the mean current consumed by the commutation device or the inverter.
The inventors have, however, ascertained that such a process presents drawbacks when it is a question of controlling the displacement of an operating lever of a throttle, for example.
In fact, in the case of a lever linked to a friction brake the load of the motor is almost constant, whatever the speed of displacement of the lever (and therefore the speed of rotation of the motor). Consequently, the conventional process for regulating the speed of the motor is not very robust and exhibits a large scattering, the current being substantially constant over the entire range of variation of speed of displacement of the lever.